The conventional incandescent lamp used for home lighting is commonly known as the A-line gas filled lamp having a wattage between about 40 and 150 watts. The construction parameters for such lamps are substantially uniform among different manufacturers, the lamps being designed for operation from a nominal voltage of 120 volts, 60 cycle AC with standard power ratings of 40, 60, 75, 100 and 150 watts. Such lamps are rated for average operating life and for standard lamps a rated life of 1,000 hours or less is typical. For convenience, these may be termed short-life lamps as compared with known lamps having extended life ranging upward from 1,000 hours to several years. In general, life of a given lamp may be increased by operating the tungsten filament at a reduced temperature. This can be achieved by changing the design of the filament. A typical 100 watt lamp has a coiled tungsten wire filament of 0.0635 mm diameter and an uncoiled length of 577 mm. It has an operating temperature of 2577.degree. C. and an average operating life of 750 hours. The life of this lamp could be increased while retaining the same wattage rating by changing the filament configuration so that it operates at a lower temperature; however, the efficiency in terms of lumens per watt would be decreased.
Another way of extending the operating life of an incandescent lamp is to reduce the effective voltage across the filament. This has been done in the prior art by connecting a rectifying diode series with the filament so that only alternate half cycles of the supply voltage are applied to the filament. This has the effect of reducing the time average value of voltage to about 0.7 of that produced by the unrectified supply voltage. Consequently, the filament is operated at a substantially lower temperature and also at a lower efficiency in terms of lumens per watt. The following patents describe the use of a rectifying diode in series with the filament of an incandescent lamp, the diode being installed in a socket for a lamp:
______________________________________ Muneson 3,450,893 Beining 3,963,956 Borneman 3,823,339 ______________________________________
The following patents describe incandescent lamps with a diode incorporated into the structure of the lamp:
______________________________________ Pearson 3,148,305 Anderson et al 3,869,631 ______________________________________
One difficulty encountered in the use of a diode for extending lamp life is that the reduction of effective voltage without a change in the filament design results in large decrease in filament temperature and the light output of the lamp is substantially diminished. Operation at the reduced temperature also results in lower efficiency of the filament. For example, the standard 100 watt lamp mentioned above has an operating temperature of 2577.degree. C. without a diode and the light output is 1750 lumens. With a diode in this lamp in series with the filament, the operating temperature is reduced to 2164.degree. C. and the light output is 510 lumens. The diode has the effect of reducing power consumption of the lamp from 100 watts to 58.8 watts. The average life for the lamp without the diode is 750 hours and the same lamp with a diode is 8.6 years.
The Anderson et al patent cited above attempts to overcome the disadvantages of an incandescent lamp with a diode in series with the filament. The lamp of this patent utilizes a filament made of tungsten wire having a weight of about 50 percent greater than is normally used for the particular lamp under consideration. The lamp is operated at a filament temperature of at least 2600.degree. K. According to the patent, this modification of the lamp restores it to its rated wattage and improves the efficiency in terms of lumens per watt, as compared with the lamp having an unmodified filament and a diode in circuit with the filament.
In the prior art, it is known to use a heat reflector for the purpose of reflecting infrared radiation back to the filament to reduce the energy loss in lamp operation. Such arrangements are described in the following patents:
______________________________________ Hoffman 1,425,967 Freeman 3,209,188 Florington et al 4,160,929 ______________________________________
In the lamps described in these patents, the envelope is provided with a layer of material which is substantially transparent for visible light and highly reflective for infrared energy. The Florington et al patent is of great interest in that it discloses a shield of reflective material mounted on the stem of the lamp for the purpose of reflecting infrared energy back to the filament.
A general object of this invention is to overcome certain disadvantages of the prior art and to provide an improved incandescent lamp having an extended operating life and increased efficiency.